How to access the window width/height inside a GLSL Shader?
It depends if the shader is set as an actuator "Filter 2D" or not. Then you can use the predefined uniform floats bgl_RenderedTextureWidth and bgl_RenderedTextureHeight. If it is just an arbitrary shader you would need to pass the values yourself as uniforms e.g.:

// glsl
uniform float ww;
uniform float wh;
...


And pass the values via the python api like:

# python
shader.setUniform1f("ww", float(bge.render.getWindowWidth()))
shader.setUniform1f("wh", float(bge.render.getWindowHeight()))


To set up the shaders you usually use code like this:

cont = bge.logic.getCurrentController()
own = cont.owner

vs = """ 
// vertex shader code here
"""

fs = """ 
// fragment shader code here
"""

mesh = own.meshes[0]
for mat in mesh.materials:
  shader = mat.getShader()
  if shader != None:
    if not shader.isValid():
        shader.setSource(vs, fs, True)
    shader.setUniform...